The invention relates to a drive having an energy recovery and retarder function.
In utility vehicles, such as, for example, construction site vehicles or fork lift trucks, the vehicle is often accelerated and subsequently braked again during working operation. In addition to the primary drive, working hydraulic units are provided in most cases and are operated by means of an additional hydrostatic piston engine. For the driving operation, these hydrostatic piston engines can be connected to the drive and consequently to the primary energy source of the drive. In order to store the energy which is released during the braking of either an operating device or the travel drive, it is known from DE 32 47 335 C2 to connect a store to the hydrostatic piston engine. By charging the store, kinetic energy is converted into pressure energy. To this end, the hydrostatic piston engine draws pressure medium from a pressure medium reservoir and conveys it into the store with the pressure being increased. Owing to the increasing pressure as the pressure medium is conveyed, the vehicle or the operating device is subjected to a braking effect. In addition, it is possible to use a pressure limitation valve in order to brake the operating device or the vehicle. The pressure medium conveyed by the hydrostatic piston engine is depressurised via the pressure limitation valve.
The drive known from DE 32 47 335 C2 has the disadvantage that, for example, in the event of a relatively long hill start, the capacity of the store is reached. Further braking of the vehicle or only maintaining the selected travel speed by conveying pressure medium counter to the pressure in the store then becomes impossible. In order to achieve an adequate braking effect, the corresponding kinetic energy at the pressure limitation valve must consequently be converted into heat. This leads to a significant rise in the temperature of the pressure medium.